On the basis of compartmentalization, the biosynthesis of fatty acids and triacylglycerides may be regarded as separate pathways of biosynthesis, but in view of the end product, as one pathway of biosynthesis. De novo biosynthesis of fatty acids takes place in plastids and is catalyzed by three enzymes or enzyme systems, namely (1) acetyl-CoA carboxylase (ACCase), (2) fatty acid synthase (FAS), and (3) acyl-[ACP] thioesterase (TE). In most organisms the end products of these reaction sequences are either palmitic acid, stearic acid or, after desaturation, oleic acid.
Fatty acid synthase consists of an enzyme complex of dissociable single enzymes with the individual enzymes acetyl-[ACP] transacylase, malonyl-[ACP] transacylase, .beta.-ketoacyl-[ACP] synthases I, II, III, .beta.-ketoacyl-[ACP] reductase, liydroxyacyl-[ACP] dehydratase, enoyl-[ACP] reductase and ACP=acyl carrier protein.
Then, in the so-called Kennedy pathway, triacylglyceride biosynthesis takes place from glycerol 3-phosphate and fatty acids--which are present as acyl-CoA substrates--in the cytoplasm at the endoplasmic reticulum.
The expression of genes of fatty acid biosynthesis is decisively regulated by their upstream promoters. They control the strength of the tissue-specific, development-specific or external stimulus-induced expression of the genes downstream thereto.
A large number of plant promoters, including seed-specific promoters, have been isolated and characterized during the last few years. A few examples are the HMW promoter (L. S. Robert et al., Plant Cell 1, pp. 569-578 (1989); V. Colot et al., Mol. Gen. Genet. 216, pp. 81-90 (1989)), Baumlein et al., The Plant Journal 2, pp. 233-239, 1992; zein promoter (A. J. M. Matzke et al., Plant Mol. Biol. 14, pp. 323-332 (1990)), lectin promoter (P. Guerche et al., Mol. Gen. Genet. pp. 306-314 (1990), USP promoter (H. Baumlein et al., Mol. Gen. Genet. 225, pp. 459-467 (1991)), napin promoter (M. Stayton et al., Aust. J. Plant Physiol. 18, pp. 507-517 (1991), oleosin promoter (J. S. Keddie et al. Plant Mol. Biol. 19, pp. 443-453 (1992)) or ACP promoter (J. de Silva et al., Plant Mol. Biol. 18, pp. 1163-1172 (1992)). The extent to which they are suitable for the expression of a given gene, and the differences which they show with regard to the desired phenotype, cannot be predicted. Frequently the studies of their specificity were carried out on plant species other than the respective cultivated plants of interest. Investigated in rape and found suitable for modifications of the fatty acid metabolism were a napin promoter (J.C. Kridl et al., Seed Sci. Res. 1, pp. 209-219 (1991), D.S. Knutzon et al., Proc. Natl. Acad. Sci. 89, pp. 2624-2628 (1992), and an ACP promoter (Knutzon et al., D. E. Scherer et al., Plant. Mol. Biol. 9, pp. 127-134 (1987)).